Prepared by Murdoch University for the Swan River Trust, the report shows 36 dolphins used the river in 2011-12.

It found 16 adults and four calves lived there most of the year, although they also ranged into coastal areas.

A further five adults and two calves made regular visits to the river, while six adults and three calves visited occasionally.

Advertisement

Environment Minister Bill Marmion said the research offered valuable insight into dolphin society, finding that both males and females formed relationships with the same sex, but that the male alliances appeared stronger.

"Male dolphins tend to associate as constant companions, while the females have a looser network of casual acquaintances with other females," he said.

"Two visiting males, Backpack and Fingers, have been constant companions for more than 20 years.

"The report found the resident dolphins associate preferentially with each other in distinct groups, but they also form consort relationships with visiting dolphins.

"At least two visiting females with calves were in consort relationships with resident males."

Mr Marmion said the report also showed the population had recovered well from 2009 when six dolphins died in the river.

He said it completed a series of studies into the deaths and confirmed that cetacean morbillivirus, which weakened the immune system, played a role in the deaths.

The report also noted an increase in deaths and strandings of whales and dolphins reported along the WA coast that year, indicating the virus may have affected other cetacean populations in 2009.

Researchers also documented the first case of a third generation of river dolphin.

Resident dolphin, Moon, who was first recorded with her mother Socket in 2001, now has her own calf named Night, aged about three.